1. Field of the Invention
This invention relates to frictional locking couplings used, for example, in mechanical power transmission systems to fasten two shafts together, or to fasten two components to one another in such a manner as to resist axial thrust, or even a combination of the two.
2. Description of the Prior Art
For many years, in the field of mechanical power transmission, the use of keys and keyways has been accepted practice, although the severe stress concentration in shafts due to keyseats has long been recognized as one of the prime causes of failure due to fatigue. Furthermore, press-fitting is often employed in conjunction with a key and this accentuates the problems of mounting and dismounting whilst compounding the effect of stress concentration.
In recent years several types of devices have become available which overcome some of the problems outlined above. Most of these devices use interlocking conical tapers which are actuated by means of a ring of screws; these screws must all be tightened in a correct sequence and to a specified torque setting to ensure satisfactory torque-transmitting capacity on the part of the joined components. For small-diameter shafts and light-duty joints this may well be acceptable; however, on large heavy-duty applications the process of tightening the screws becomes extremely tedious and time-consuming. In the course of tightening a single joint it is not uncommon to apply a torque-wrench ten or twelve times to each of perhaps two dozen screws before finally achieving the condition when all are fully tight. If any one screw is found to require further tightening, then the process of tightening this one screw will also necessitate going around all the remaining screws because these will very probably have become loosened as the conical tapers move. Furthermore, the heads of the screws lie in close proximity to the surface of the shaft and this makes it difficult to apply the torque-wrench; in some instances, a special torque-wrench with truncated head is necessary to gain access to the screw heads. The time factor alone makes the fitting process very expensive for practical engineering purposes.
Another method of achieving a joint which is suitable for the transmission of mechanical power and which is readily dismountable is the oil-injection method. This entails injecting oil at very high pressure between mounted components, the oil pressure being slightly in excess of the surface pressure of the interference fit. To facilitate manufacture, the mating surfaces are usually slightly tapered. This is a very reliable method, well-proven in heavy engineering over many years. There are, however, several drawbacks. Firstly, the tapered surfaces are difficult to manufacture accurately with the necessary oil-feed and drainage ducts. Secondly, the finely tapered surfaces render it impossible to achieve accurate axial positioning of the mounted components without using an intermediate sleeve. Thirdly, special external hydraulic tooling has to be provided to supply axial thrust to the joint whilst fitting; normally, two separate hydraulic systems are used, one for radial pressurisation and the other for axial pressurisation.